South Korea’s prime minister promised on Monday to “accurately reveal” the truth behind allegations that the domestic spy service interfered last year’s presidential election.

Opposition parties have spent weeks hammering President Park Geun-hye’s administration over the issue, suggesting that it calls the legitimacy of her December poll victory into question.

“The government will accurately reveal what happened and what caused it,” Prime Minister Chung Hong-won said in a public statement.

“The government will not hesitate to take necessary actions” against anyone found responsible of wrongdoing, he added.

The government will accurately reveal what happened and what caused it

South Korean Prime Minister Chung Hong-Won

The allegations focus on the origin of multiple online messages, posted prior to polling day, that denounced opposition presidential candidate Moon Jae-in as a North Korean sympathiser, while extolling the virtues of ruling party candidate Park.

While initially dismissed as the work of a couple of maverick National Intelligence Service (NIS) agents, further investigations have suggested a wider, coordinated smear campaign.

Park has denied any involvement, and Chung stressed that the president wanted a complete investigation.

“The president has made it clear from the outset that she did not get any help from the intelligence agency during the election,” he said.

“She also has said her government will carry out reform of the NIS more thoroughly than any other former governments,” he added.

A senior prosecutor who had led the probe into the NIS until his recent dismissal, told a parliamentary committee hearing last week that he had come under pressure to soft-pedal his investigation.

North Korea has gleefully pounced on the allegations of electoral malpractice, with the ruling party newspaper Rodong Sinmun commenting at the weekend on the “plot-breeding organs” in Seoul “which work with blood-shot eyes to violate democracy.”